1. Field of the Invention
The invention relates to a once-through steam generator including a gas flue formed of steam generator tubes welded to one another in a gas-tight manner through fins, the steam generator tubes connected in parallel for the throughflow of a flow medium, having a surface structure on their inside for generating a high heat transfer from their inner wall surface to the flow medium and disposed approximately in a spiral coil in a firing region of the gas flue.
Smooth tubes are normally used in the combustion chamber walls of a once-through steam generator which are composed of spirally disposed-steam generator tubes in a firing region. However, a configuration of that type is only suitable for load states of normally more than about 40% of the design load of the once-through steam generator due to the heat transfer properties of a smooth tube when the flow velocity of a medium flowing in it is low. Below that minimum load of about 40% of the design load, a circulating mode is normally superposed on the once-through mode of the steam generator, so as to ensure sufficient cooling of the steam generator tubes. However, switching to a circulating mode in that way leads to a lowering of the fresh-steam temperature of the once-through steam generator of about 80.degree. C.
However, it may be necessary to construct the once-through steam generator in such a way that it can be operated at a sufficiently high fresh-steam temperature even in load states of more than 20% of the design load. That is done particularly in order to avoid nighttime shutdowns of a power plant supplied by the once-through steam generator. That is made possible by the use of those steam generator tubes which have a surface structure on their inside, for example in the form of ribs (internally ribbed tubes), for generating a high heat transfer from their inner wall surface to the flow medium. The use of such steam generator tubes with internal ribbing in a once-through steam generator having vertically disposed evaporator tubes is known, for example, from Published European Patent Application 0 503 116 Al, corresponding to U.S. application Ser. No. 08/151,257, filed Nov. 12, 1993. However, a steam generator tube with internal ribbing has a markedly higher pressure loss due to friction in comparison with a smooth tube. In the case of multiple heating of a steam generator tube, such an increased pressure loss due to friction may lead to temperature differences between adjacent tubes, in particular at an outlet of an evaporator. That is due to fluctuations in the mass flow of the medium flowing through the tube. Those temperature differences may cause damage due to inadmissible thermal stresses.